Something to remember when you go on "field trips" as my boss calls them; Shopping centers, etc.
Standing out front of a place like Target/Safeway or whatever your big grocery stores are called are great exposure to weird things.
Five to seven minutes is not too long to start. You want to work up to a calm acceptance. You don't want a panicky dog, and they are good places to work up your confidence as a handler. Don't go places where you are out of your comfort zone as the dog will pick up on it.
We are pretty lucky because we live really close to our big county hospital. Alice got to see ambulances, firetrucks (on full siren mode) people in walkers, wheelchairs, sheriff's car and because of where we live people from other cultures that to a dog dress REALLY scary. Turbans, saris and sarongs.
Alice actually got to sit in the back of a retired K-9s car and now she thinks ALL the sheriffs will let her do it...
My rambling point is: What are your goals for you and your dog's relationship?
Personally, I like to have a confident, happy, mentally balanced dog.
It's ok to get startled, but how fast she recovers is what I'm looking for.